The present invention relates to a thick film resistor type printed circuit board, which is used for a hybrid integrated circuit requiring high reliability.
FIGS. 1a to 1d show one example of manufacturing steps of a known thick film resistor type printed circuit board. As shown in FIG. 1a, a plurality of wiring conductive layers 14 made of copper are initially formed on an insulating substrate 1 by chemical etching of a copper foil, by printing and curing of copper paste, etc. Then, as shown in FIG. 1b, silver paste is printed and cured so as to partially overlap the wiring conductive layers 14 such that a plurality of pairs of terminal electrode layers 12 and 12' are formed. Each pair of the terminal electrode layers 12 and 12' are so provided as to confront each other. Subsequently, as shown in FIG. 1c, a plurality of resistor layers 13 each connecting each pair of the terminal electrode layers 12 and 12' is formed by printing, drying and then, curing at about 100.degree. to 200.degree. C. resistor paste made of carbon resin, etc. Thereafter, as shown in FIG. 1d, cuts 5 are formed on the resistor layers 13 by laser beams, etc. so as to adjust resistance values of the resistor layers 13. Furthermore, resinous paste is printed at other portions of the insulating substrate 1 than a joint portion to be connected to an external circuit of the wiring conductive layers 14 and is cured through heating, irradiation of ultraviolet rays thereto, etc. so as to form a protective layer 6.
As described above, in the known thick film resistor type printed circuit board, the terminal electrode layers 12 and 12', acting as a resistor, are made of silver. Thus, in the case where a potential difference is produced between the terminal electrode layers 12 and 12' during actual use of the known thick film resistor type printed circuit board, a phenomenon of so-called silver migration takes place in which silver ions (Ag.sup.+) in the terminal electrode layers 12 and 12' migrate in the presence of moisture acting as an electrolyte such that silver is deposited on one of the terminal electrode layers 12 and 12' having a lower electric potential, thereby resulting in a short-circuit between the terminal electrode layers 12 and 12' in the worse case.